U.S. patent number 4,387,969 [Application Number 06/222,874] was granted by the patent office on 1983-06-14 for single-lens reflex optical system for endoscopes.
This patent grant is currently assigned to Olympus Optical Co., Ltd.. Invention is credited to Kimihiko Nishioka, Nobuo Yamasita.
United States Patent |
4,387,969 |
Nishioka , et al. |
June 14, 1983 |
Single-lens reflex optical system for endoscopes
Abstract
A single-lens reflex optical system for endoscopes comprising an
observing optical system, a light-splitting means and a
photographing optical system, the observing optical system
comprising a diverging front lens group arranged on a first optical
axis extending in parallel with the longitudinal direction of an
endoscope and having negative refractive power, a positive lens
group arranged on the first optical axis, and a first converging
rear lens group arranged on the first optical axis and having
positive refractive power, the light-splitting means arranged
between the positive lens group and first converging rear lens
group, the photographing optical system comprising the diverging
front lens group, the positive lens group, and a second converging
rear lens group arranged on a second optical axis deflected by the
light-splitting means and having positive refractive power, the
single-lens reflex optical system enabling to make diameters of
lenses small and, consequently, enabling to make the diameter of
distal end of endoscope small.
Inventors: |
Nishioka; Kimihiko (Hachiouji,
JP), Yamasita; Nobuo (Hachiouji, JP) |
Assignee: |
Olympus Optical Co., Ltd.
(Tokyo, JP)
|
Family
ID: |
11473561 |
Appl.
No.: |
06/222,874 |
Filed: |
January 6, 1981 |
Foreign Application Priority Data
Current U.S.
Class: |
359/753; 359/726;
385/117; 385/119 |
Current CPC
Class: |
G03B
17/48 (20130101); G02B 23/2407 (20130101) |
Current International
Class: |
G02B
23/24 (20060101); G03B 17/48 (20060101); G02B
005/17 () |
Field of
Search: |
;350/415,445,447,469,474,173,96.26 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Corbin; John K.
Assistant Examiner: Dzierzynski; P.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. A single-lens reflex optical system for endoscopes comprising an
observing optical system, a light-splitting means and a
photographing optical system, said observing optical system
comprising a diverging front lens group arranged on a first optical
axis extending in parallel with the longitudinal direction of an
endoscope and having negative refractive power, a positive lens
group arranged on said first optical axis, a first converging rear
lens group arranged on said first optical axis and having positive
refractive power, and a first aperture stop arranged between said
positive lens group and said first converging rear lens group and
on said first optical axis, said light-splitting means being
arranged between said positive lens group and said first converging
rear lens group, said photographing optical system comprising said
diverging front lens group, said positive lens group, a second
converging rear lens group arranged on a second optical axis split
by said light-splitting means and having positive refractive power,
and a second aperture stop arranged between said positive lens
group and said second converging rear lens group, said optical
system satisfying the following conditions: ##EQU6## wherein
reference symbol f represents the focal length of the photographing
optical system, reference symbol f.sub.1 represents the focal
length of the diverging front lens group, reference symbol f.sub.2
represents the focal length of the positive lens group, reference
symbol A represents the distance between the diverging front lens
group and the second aperture stop, reference symbol t.sub.2
represents the distance between the positive lens group and the
second aperture stop, reference symbol F represents the F-number of
the photographing optical system, reference symbol .omega.
represents a half field angle of the photographing optical system,
and reference symbol k represents the ratio between the height of
marginal ray in the photographing optical system at the front of
the light-splitting means and the height of marginal ray in the
observing optical system at the front of the light-splitting
means.
2. A single-lens reflex optical system for endoscopes according to
claim 1 further comprising a glass block arranged between said
diverging front lens group and said positive lens group.
3. A single-lens reflex optical system for endoscopes according to
claim 2 wherein said positive lens group comprises a biconvex lens
of which the absolute value of the radius of curvature of the
surface on the object side is smaller than the absolute value of
the radius of curvature of the surface on the image side, and
wherein said first converging rear group comprises a cemented
doublet and said second converging rear lens group comprises a
cemented doublet.
4. A single-lens reflex optical system for endoscopes according to
claim 3, in which said single-lens reflex optical system for
endoscopes has the following numerical data:
wherein reference symbols r.sub.1 through r.sub.17 respectively
represent radii of curvature of respective surfaces of respective
lenses and of the prism and glass block, reference symbols d.sub.1
through d.sub.17 respectively represent distances between
respective surfaces, reference symbols n.sub.1 through n.sub.10
respectively represent refractive indices of respective lenses and
of the prism and glass block, and reference symbols .nu..sub.1
through .nu..sub.10 respectively represent Abbe's numbers of
respective lenses and of the prism and glass block.
Description
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a single-lens reflex optical
system for endoscopes.
(b) Description of the Prior Art
To observe the inside of tubular members such as a rectum,
esophagus, etc., a forward-viewing type endoscope is used. This is
due to the following reason. If a side-viewing type endoscope is
used for observation of a tubular member, the object to be observed
or photographed (the inner wall of the esophagus or the like) comes
into close contact with the cover glass arranged at the front of
the objective of the endoscope and, consequently, it becomes
impossible to observe or photograph the object with an adequate
magnification and field.
FIG. 1 shows an example of known single-lens reflex optical systems
to be used in a forward-viewing type endoscope, which is used for
observation of a tubular member and which is arranged so that a
film cassette can be loaded. The optical system shown in FIG. 1
comprises a photographing optical system and an observing optical
system, the photographing optical system comprising lens components
2, 6 and 7 arranged on the optical axis 1 which extends in parallel
with the longitudinal direction of the endoscope, the observing
optical system comprising lens components 2, 3 and 4, the lens
components 3 and 4 being arranged on the optical axis 1' which is
split by a light-splitting prism 9 and directed to an optical fiber
bundle 5. In case of the above-mentioned optical system, a film
surface 8 is arranged perpendicular to the optical axis 1.
Therefore, the film cassette should be loaded in the direction
perpendicular to the longitudinal direction of the endoscope. As,
however, the film cassette is very long compared with the diameters
of lens components, the diameter of the distal end of the endoscope
becomes large when the film cassette is arranged in the direction
perpendicular to the longitudinal direction of the endoscope.
As a method to eliminate the above-mentioned disadvantage, it may
be considered to arrange the film cassette in the direction
parallel with the longitudinal direction of the endoscope. FIG. 2
shows an optical system in which the above-mentioned idea is
adopted. The optical system shown in FIG. 2 comprises an observing
optical system and a photographing optical system, the observing
optical system comprising a diverging front lens group 11 and a
first converging rear lens group 12 which are arranged on the
optical axis 10, the photographing optical system comprising the
diverging front lens group 11 and a second converging rear lens
group 14, the second converging rear lens group 14 being arranged
on the optical axis 10' which is split by a light-splitting prism
13.
In case of the optical system shown in FIG. 2, the space in the
diametral direction in the distal end of the endoscope which is
occupied by the film cassette 15 becomes very small compared with
the case of the optical system shown in FIG. 1. However, in case of
a retrofocus-type optical system for endoscopes as shown in FIG. 2,
the airspace between the diverging front lens group and each of the
converging rear lens groups becomes large when the film cassette is
arranged in the longitudinal direction of the endoscope. In case of
a retrofocus-type optical system, paraxial rays are diverged by the
diverging front lens group and, consequently, the height of the
paraxial marginal ray which enters the converging rear lens group
becomes larger when the airspace between the front and rear lens
groups becomes larger. Therefore, when the airspace between the
diverging front lens group and converging rear lens group becomes
larger, diameters of lenses in the converging rear lens group
should be made larger. Besides, in order to let the offaxial ray
enter the coverging rear lens group at a pre-determined angle, the
height of the offaxial ray which enters the diverging front lens
group should be made larger when the airspace between the diverging
front lens group and converging rear lens group becomes larger.
Consequently, it becomes necessary to make the diameter of the lens
constituting the diverging front lens group larger. As explained in
the above, when the airspace between the diverging front lens group
and converging rear lens group becomes large due to the fact that
the film cassette 15 is arranged in the longitudinal direction of
the endoscope, diameters of lenses in both of the diverging front
lens group and converging rear lens group become large. In other
words, in spite of the fact that the space in the distal end of the
endoscope occupied by the film cassette 15 becomes smaller,
diameter of lenses become larger and, consequently, the diameter of
the distal end of the endoscope becomes larger also in case of the
optical system shown in FIG. 2.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to
provide a single-lens reflex optical system for endoscopes which is
arranged so that a film cassette can be loaded in the direction
parallel with the longitudinal direction of the endoscope and, at
the same time, arranged so that diameters of lenses constituting
the optical system will not become large by arranging a positive
lens group between a diverging front lens group and converging rear
lens group.
Another object of the present invention is to provide a single-lens
reflex optical system for endoscopes comprising an observing
optical system and a photographing optical system, said observing
optical system comprising a diverging front lens group, a positive
lens group, and a first converging rear lens group, said
photographing lens group comprising said diverging front lens
group, said positive lens group, and a second converging rear lens
group, said second converging rear lens group being arranged on an
optical axis which is deflected by a light-splitting means, said
single-lens reflex optical system for endoscopes satisfying the
following conditions: ##EQU1## wherein reference symbol f
represents the focal length of the photographing optical system,
reference symbol f.sub.1 represents the focal length of the
diverging front lens group, reference symbol f.sub.2 represents the
focal length of the positive lens group, reference symbol A
represents the distance between the diverging front lens group and
aperture stop of the photographing optical system, reference symbol
t.sub.2 represents the distance between the positive lens group and
said aperture stop, reference symbol F represents the F-number of
the photographing optical system, reference symbol .omega.
represents a half filed angle of the photographing optical system,
and reference symbol k represents the ratio between the height of
marginal ray in the photographing optical system at the front of
the light-splitting means and the height of marginal ray in the
observing optical system at the front of the light-splitting
means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic view of a known single-lens reflex optical
system for a forward-viewing endoscope;
FIG. 2 shows a schematic view of a single-lens reflex optical
system wherein a film cassette is arranged in the distal end of an
endoscope so that the film cassette is located in parallel with the
longitudinal direction of the endoscope;
FIG. 3 shows a sectional view of an embodiment of the single-lens
reflex optical system for endoscopes according to the present
invention;
FIG. 4 shows a front view of a light-splitting surface of a
light-splitting prism used in said embodiment; and
FIG. 5 shows a diagrammatic illustration for explaining the theory
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 3 shows an embodiment of the single-lens reflex optical system
for endoscopes according to the present invention. In this figure,
numeral 17 designates a diverging front lens group comprising a
negative lens which serves also as a cover glass, numeral 18
designates a glass block with parallel plane surfaces, and numeral
19 designates a positive lens group (biconvex single lens). The
positive lens group 19 is so arranged that the absolute value of
the radius of curvature of its surface on the object side is
smaller than the absolute value of the radius of curvature of its
surface on the image side. Numeral 20 designates a light-splitting
prism, numeral 23 designates a first converging rear lens group
comprising a cemented doublet 21, which consists of a negative lens
and positive lens, and a positive lens 22. The diverging front lens
group 17, glass block 18, positive lens group 19, light-splitting
prism 20 and the first converging rear lens group 23 are
respectively arranged on a first optical axis 16, which extends in
parallel with the longitudinal direction of an endoscope, and
constitute an observing optical system. Numeral 25 designates a
second converging rear lens group arranged on a second optical axis
24, which is deflected by the light-splitting prism 20, and
comprising a cemented doublet consisting of a positive lens and
negative lens. The diverging front lens group 17, glass block 18,
positive lens group 19, light-splitting prism 20 and the second
converging rear lens group 25 constitute a photographing optical
system. The above-mentioned observing optical system forms an image
of an object, which is not shown, onto an image-transmitting
optical fiber bundle 26 while the above-mentioned photographing
optical system forms an image of the object onto a film surface
27.
FIG. 4 shows a front view of a light-splitting surface 28 of the
light-splitting prism 20. As shown in this figure, the surface 28
is composed of a central portion 29, intermediate portion 31 and
outer portion 30, the central portion 29 being a circular
reflecting surface, the intermediate portion 31 being an annular
transparent portion, the outer portion 30 being a light shielding
surface. Therefore, a portion of light which enters the
light-splitting prism 20 is reflected by the central portion 29 and
reaches the film surface 27 passing through the second converging
rear lens group 25. The remainder of light which enters the prism
20 passes through the intermediate portion 31 and reaches the end
face of the optical fiber bundle 26 passing through the first
converging rear lens group 23. In other words, the central portion
29 of the surface 28 serves as the aperture stop for the
photographing optical system while the intermediate portion 31
serves as the aperture stop for the observing optical system.
The optical system according to the embodiment described in the
above enables it to attain the object of the present invention.
However, it is possible to provide a more favourable optical system
by arranging so that the above-mentioned optical system satisfies
the afore-mentioned conditions. This is described below based on
the paraxial theory assuming that each of lens groups constituting
the optical system is a thin lens.
FIG. 5 shows a diagrammatic illustration of the portion on the
object side of the light splitting surface 28, i.e., stop surface
of the photographing optical system, of the signle-lens reflex
optical system according to the embodiment shown in FIG. 3. In this
figure, it is possible to express the height h of the principal ray
l.sub.1 of the offaxial rays which enters the diverging front lens
group 17 at the maximum field angle is expressed by the following
formula (1). ##EQU2## In the above formula (1), reference symbol
.omega. represents the half field angle, reference symbol f.sub.1
represents the focal length (negative value) of the diverging front
lens group 17, reference symbol f.sub.2 represents the focal length
of the positive lens group 19, reference symbol A represents the
distance from the diverging front lens group 17 to the stop surface
28 of the photographing optical system, and reference symbol
t.sub.2 represents the distance from the positive lens group 19 to
the stop surface 28.
In case of the embodiment shown in FIG. 3, the second optical axis
24 directed from the aperture stop of the photographing optical
system to the second rear lens group 25 reaches the center of the
film cassette and, generally, the length of the film cassette is
about 4.5 times of the image height C on the film surface.
Therefore, the distance A and image height C should have the
relation expressed by the following formula.
Besides, when the focal length of the photographing optical system
is represented by a reference symbol f, the focal length f and
image height C have the relation expressed by the following formula
(2).
To make the lens diameter of the diverging front lens group 17
small, it is preferable to arrange so that the relation expressed
by the formula h.ltoreq.C is satisfied. From this relation and
formulas (1) and (2), the following formula (I) is obtained.
##EQU3## That is, when it is so arranged that the above formula (I)
is satisfied, it is possible to make the lens diameter of the
diverging front lens group small.
Now, the condition for making the lens diameter of the converging
rear lens group small is obtained as described below. To prevent
the diameters of lenses on the image side of the positive lens
group 19 from becoming large, it is necessary to arrange that
paraxial rays will not diverge in the portion on the image side of
the positive lens group 19. This can be attained when the following
formula (II) is satisfied.
To make the lens diameter of the converging rear lens group still
smaller, it is desirable to arrange that the height H.sub.F of the
paraxial marginal ray l.sub.2 which enters the positive lens group
19 will not exceed the image height C. This is due to the following
reason. That is, when the refractive power of the diverging front
lens group 17 is made stronger as shown by the formula (I), the
height H.sub.F of the paraxial marginal ray l.sub.2 becomes higher.
Consequently, it is necessary to make the lens diameter larger and,
moreover, the frames holding the respective lenses and
light-splitting prism will interfere with each other. This will be
prevented when the height of the observing ray is made small
because the diameter of the observing light pencil on the stop
surface 28 is larger than the diameter of the photographing light
pencil on the stop surface 28. As the ratio k between the heights
of rays in the observing optical system and photographing optical
system is constant at positions just in front of the converging
rear lens groups, the height H.sub.F of the paraxial marginal ray
l.sub.2 which enters the positive lens group 19 can be expressed by
the following formula. ##EQU4## In the above formula, reference
symbol F represents the F-number of the photographing optical
system. As the relation between H.sub.F and C is H.sub.F .ltoreq.C,
the following formula (III) is obtained from this relation and
formula (2). ##EQU5##
When it is so arranged that the focal length of the positive lens
group, etc. satisfy the above-mentioned formulas (II) and (III),
the height of ray l.sub.2 which enters the positive lens group 19
becomes the maximum and, therefore, diameters of lenses
constituting the converging rear lens groups, which are arranged
behind the positive lens group, become small. Besides, in the
photographing optical system, the angle between the offaxial
principal ray and second optical axis 24 is large as it is evident
from FIG. 3, the second converging rear lens group becomes still
smaller. Moreover, the end face of the image-transmitting optical
fiber bundle 26 is generally smaller than the film surface, it is
possible to make the first converging rear lens group small.
As explained hitherto, the present invention enables to make the
overall diameter of optical system for endoscopes small. For an
actual optical system, however, aberrations thereof should be
corrected favourably. In case of the retrofocus-type optical system
shown in FIG. 3, the diverging front lens group 17 refracts the
offaxial lower ray (ray far from the optical axis) more strongly
compared with the paraxial upper ray (ray near the optical axis).
As a result, coma becomes asymmetrical. To correct spherical
aberration and to prevent coma from occurring, in case of the
embodiment shown in FIG. 3, the positive lens group 19 is arranged
that the absolute value of the radius of curvature of its surface
on the object side becomes smaller than the absolute value of the
radius of curvature of its surface on the image side. Besides, the
cemented doublets 21 and 25 are arranged in the converging rear
lens groups so that asymmetry of coma is corrected more favourably
and chromatic aberration is also corrected satisfactorily
favourably. Moreover, the glass block 18 inserted between the
diverging front lens group 17 and positive lens group 19 is
effective for making the height of ray in the diverging front lens
group small.
An example of numerical data of the embodiment shown in FIG. 3 is
as shown below.
______________________________________ r.sub.1 = .infin. d.sub.1 =
0.9 n.sub.1 = 1.58921 .nu..sub.1 = 41.08 r.sub.2 = 2.498 d.sub.2 =
0.95 r.sub.3 = .infin. d.sub.3 = 6.0 n.sub.2 = 1.883 .nu..sub.2 =
40.76 r.sub.4 = .infin. d.sub.4 = 0.55 r.sub.5 = 5.288 d.sub.5 =
1.2 n.sub.3 = 1.6968 .nu..sub.3 = 55.52 r.sub.6 = -11.914 d.sub.6 =
0.5 r.sub.7 = .infin. d.sub.7 = 1.1 n.sub.4 = 1.8061 .nu..sub.4 =
40.95 stop (28) d.sub.8 = 1.1 n.sub.4 = 1.8061 .nu..sub.4 = 40.95
r.sub.8 = .infin. d.sub.9 = 1.6 r.sub.9 = 4.68 d.sub.10 = 1.35
n.sub.5 = 1.618 .nu..sub.5 = 63.38 r.sub.10 = -2.409 d.sub.11 =
0.45 n.sub.6 = 1.834 .nu..sub.6 = 37.19 r.sub.11 = .infin. d.sub.12
= 1.1 n.sub.7 = 1.8061 .nu..sub.7 = 40.95 r.sub.12 = .infin.
d.sub.13 = 0.8 r.sub.13 = -12.698 d.sub.14 = 0.75 n.sub.8 = 1.74
.nu..sub.8 = 28.29 r.sub.14 = 1.799 d.sub.15 = 1.35 n.sub.9 =
1.72916 .nu..sub.9 = 54.68 r.sub.15 = -4.158 d.sub.16 = 0.24
r.sub.16 = 2.382 d.sub.17 = 1.0 n.sub.10 = 1.72916 .nu..sub.10 =
54.68 r.sub.17 = 3.428 f = 3.915 .omega. = 35.8.degree. F = 10
f.sub.1 = -4.24 t.sub.2 = 1.613 A = 6.523 f.sub.2 = 5.411 k = 1.65
C = 2.82 ##STR1## ##STR2##
______________________________________
In the above example, reference symbols r.sub.1 through r.sub.17
respectively represent radii of curvature of respective surfaces of
respective lenses and of the prism and glass block, reference
symbols d.sub.1 through d.sub.17 respectively represent distances
between respective surfaces, reference symbols n.sub.1 through
n.sub.10 respectively represent refractive indices of respective
lenses and of the prism and glass block, and reference symbols
.nu..sub.1 through .nu..sub.10 respectively represent Abbe's
numbers of respective lenses and of the prism and glass block.
In the embodiment explained hitherto, aperture stops are provided
on the light-splitting surface 28 of the light-splitting prism 20.
This arrangement enables to make the diameter of the aperture stop
of the photographing optical system small and this is preferable
for making the depth of focus on the film surface deep. In case of
the optical system having lens configuration with which the stop of
the photographing optical system is made small, it is preferable to
arrange the aperture stop of the photographing optical system at a
position in rear of the light-splitting surface. When, however, the
distance from the diverging front lens group to the aperture stop
in the photographing optical system is different from that distance
in the observing optical system, the light is partially eclipsed
and, consequently, the photographed field becomes different from
the observed field. However, when the aperture stop of the
observing optical system exists at a position between the positive
lens group and the first converging rear lens group, the
above-mentioned difference of field is small and it is not
inconvenient for practical use. If the aperture stop of the
photographing optical system comes to a position in front of the
light-splitting surface, it becomes impossible to make the diameter
of the aperture stop of the photographing optical system small and,
consequently, the depth of focus cannot be made deep. Therefore, to
simplify the composition of the optical system as a whole, it is
effective when the aperture stop of the photographing optical
system is arranged at the same position as the aperture stop of the
observing optical system.
When, the aperture stop of the photographing optical system comes
to a position in front of the positive lens group, the formulas
(I), (II) and (III) cannot be applied as they are. If, however, it
is supposed in the above case that the F-number is not changed, the
formulas (II) and (III) related to the paraxial marginal ray do not
change. Therefore, the formula (I) related to the offaxial ray can
be applied when it is supposed as f.sub.2 =.infin.. When the
aperture stops are not provided on the light-splitting surface, the
light-splitting surface may be arranged as a semitransparent
surface.
As explained hitherto based on the embodiment, the present
invention provides a single-lens reflex optical system for
endoscopes for which the outer diameter of the optical system as a
whole is made small in spite of the fact that the distance between
the diverging front lens group and converging rear lens group is
made large and, therefore, the present invention enables to make
the outer diameter of the distal end of the endoscope small.
* * * * *